The purpose of a golf swing training tool is to assist a golfer in learning the correct swing path the club head should follow during a normal golf swing, in order to consistently hit the ball in the desired direction.
In golf, the target line is an imaginary line drawn through the center of the golf ball to the intended target, generally the flag stick on the putting green of the hole being played.
It has been estimated that upwards of 75% of all golfers make the fundamental mistake of swinging the golf club across the target line in an outside-to-inside swing path. In other words, the swing path of the club head cuts across the ball, at an angle, toward the golfer's forward foot. This outside-to-inside swing path leads to shots that, for right-handed golfers, start to the left of the target line such that the ball, instead of being hit straight, is pulled, pull hooked, sliced, etc.
This swing path is the bane of most golfers, and has proven to be possibly the most difficult swing fault to correct.
Various golf training tools or devices are known that attempt to teach a golfer to swing the golf club so that the club head approaches the ball at a slight angle from inside the target line (on the side of the target line where the golfer is standing). Most devices require the golfer to hit the golf ball off the surface of the device.